This invention relates to safety brakes for stopping members of machines immediately after the members exceed predetermined safe speeds, and particularly to brakes of the type used on hoists, the brakes becoming effective as speeds of descent exceed a predetermined rate to increase braking until platforms, or cages, of the hoists are stopped.
The present improvement in safety brakes is described particularly with respect to hoists of the type used either in construction or repair of buildings. For example, typical hoists have towers constructed of lattices of steel angle iron, and the towers support along respective sides vertically movable platforms or cages for elevating workmen and materials. The cages are guided by rollers along vertical members and propelled by motors having pinions engaging vertical racks. The motors are controlled to raise or to lower the cages, and regulators associated with the drives of the motors determine usual maximum speeds. In the event that regulators associated with the motors should fail to regulate the maximum speed of the cages, safety brakes also having respective gears engaging the racks are provided to stop the cages.
Commonly safety brakes are effective only in the direction of descent to prevent dangerously fast speeds caused by weight of the cages and their loads. When the input gears of the safety brakes rotate faster than a desired maximum rate, respective centrifugal controls associated with the brakes engage respective braking members. As soon as the safety brakes begin to operate, braking force is rapidly increased until the cages are stopped. The rate in change of braking is sufficient to stop the cages quickly, but not so abrupt as to cause damage. After the brakes have been operated sufficiently to stop the cages, the brakes must obviously be reset before the cages can be moved again. To release the brakes and reset them for subsequent use, workmen usually have to be raised to the positions of the cages and use machine tools, and sometimes special tools, to reposition actuators as required for releasing the brakes and preparing them for subsequent operation.